1. Field of the Invention
The present invention relates to a yellow toner used in electrophotography, electrostatic recording, electrostatic printing or toner jet recording.
2. Description of the Related Art
In recent years, image forming apparatus that utilize electrophotography have made progress toward full-color image formation and toward digitization, and have come to be put into professional use in, e.g., design studios and be used as color copying machines for office work processing. Further, they have also come to be used as printers for computer outputting or printers for personal use.
With spread of such uses, it is sought to achieve a much higher image quality that can satisfy higher definition and higher resolution, and it is desired to make an advance of colorants for toner which have small particle diameters and a narrow particle size distribution and further have good spectral characteristics, coloring power, transparency and so forth.
Where azo pigments are used as colorants for yellow toner, it is necessary for them to be finely dispersed in mediums of various types so as to be provided with suitable properties such as coloring power and transparency. However, when made fine, the azo pigments commonly tend to cause crystal growth or transition because of their heat history or contact with solvents in the step of dispersion or in subsequent steps for toner production, so that they may bring about a problem such as a lowering of coloring power or transparency.
Examples are disclosed in which a disazo pigment C.I. Pigment Yellow 155 is used as a colorant for toner (Japanese Patents No. 3917764, No. 4011476 and No. 4092905 and Japanese Patent Applications Laid-open No. 2001-166540 and No. 2001-109196). It is disclosed that this pigment has relatively good spectral characteristics and toners can be produced by using a polymerization or pulverization process of various types. This pigment, however, is so strongly agglomerative in itself and so poorly stable in handling, in particular, in dispersion mediums, as to tend to cause a difficulty that the pigment comes to agglomerate in toners.
To deal with such a problem, diverse pigment compositions and pigment dispersants making up the compositions are proposed. For example, disclosed are i) a pigment composition made up of an azo pigment and an azo coloring matter different in type which has been added to the former as a pigment dispersant and ii) an example making use of SOLSPERSE (registered trademark; available from Lubrizol Corporation) as a pigment dispersant (Japanese Patent Application Laid-open No. 2003-128952 and International Publication No. 99/42532). These pigment composition and pigment dispersant, however, have been found not to be those which are well effectively dispersible when dispersed in organic solvents, in particular, non-polar solvents as exemplified by styrene or the like.
There is also a problem that, when used in a process for producing toner particles in an aqueous system, such as suspension granulation or suspension polymerization, the pigment dispersant migrates to the aqueous system to make the pigment low dispersible in a resin or in a polymerizable monomer (Japanese Patent Application Laid-open No. 2007-262382).
An example of a toner is disclosed which makes use of an aluminum coupling agent as a pigment dispersant (Japanese Patent Application Laid-open No. 2007-155861). In the toner obtained in this proposal, however, there is room for further improvement in regard to the achievement of both particle diameter and coloring power.